The present invention relates to an airspeed indicator for low-speed conveyances, and more particularly to an airspeed indicator for low-speed conveyances, such as hang gliders, motor hang gliders, gyrocopters, home-built airplanes, motorboats and hovercrafts, that is, other than vehicles which run on the ground.
The lowest speed at which hang gliders operate is about 25 kilometers per hour (Km/h). When the airspeed is lowered until it just exceeds the lowest limit speed, the glider loses its lift and begins to drop. In this state, it can no longer be controlled. In order to recover the glider from its stalling state, the driver lowers its nose and pushes out a control bar in the forward direction. However, recovery in this manner is not possible once the altitude of the glider becomes extremely low. For this reason, a meter such as a speedometer is essential for the driver of a hang glider.
The Pitot static pressure type speedometer has been used for measuring the airspeed of hang gliders and similar low-speed conveyances. In this method, the dynamic pressure received by a Pitot tube is fed to a diaphragm capsule included in the instruments and is indicated on an indicator which is associated with the displacement of the capsule.
The Pitot static pressure type speedometer is quite useful for relatively high-seed aircrafts, but not for low-speed conveyances such as hang gliders, whose speed is only below 200 Km/h (=125 miles/h). The reason is that at such a low speed, the pressure is very small (about 200 mm Hg or less) and therefore the instruments in which the diaphragm capsule receives the pressure do not work sufficiently and might indicate a large error. For these reasons, the Pitot speedometer is not suitable for measuring stalling speed of approximately 25 Km/h.